It is well known that a crosslinked hydrogel polymer as a water-absorbent crosslinked polymer is obtained by aqueous solution polymerization of a water-soluble ethylenically unsaturated monomer under the presence of a small amount of crosslinking agent. The crosslinked hydrogel polymer is a highly elastic semisolid gel material, and is rarely used as it is. In many cases, in order to increase the drying efficiency, the crosslinked hydrogel polymer is once crushed (granulated), and then dried and ground. Namely, the crosslinked hydrogel polymer in the form of dried powder is used as an water-absorbent resin, i.e., water-absorber.
As a method of crushing a mass of the crosslinked hydrogel polymer in the above-mentioned process, for example, it is possible to use known methods, for example, 1 crushing the crosslinked hydrogel polymer after polymerization by a screw-type extruder such as a meat chopper, 2 crushing the crosslinked hydrogel polymer during polymerization in the kneader, 3 chopping the crosslinked hydrogel polymer after polymerization with hands using scissors, and 4 cutting the crosslinked hydrogel polymer while pressing a ring-shaped cutting edge against a counter roll.
However, when crushing the crosslinked hydrogel polymer obtained by aqueous solution polymerization using a meat chopper or kneader, the crosslinked hydrogel polymer is crushed while being compressed and kneaded. Therefore, when the above-method 1 or 2 is adopted, a very strong mechanical external force acts on the crosslinked hydrogel polymer. Consequently, there is a possibility that the crosslinked polymer chain is cut, and the water-soluble component content increases.
In resent years, in order to prevent clothes, etc. from being made dirty by body exudates such as urine and blood, particulate water-absorbent resins for absorbing and holding such body exudates are widely used as a component member of sanitary materials such as paper diapers, sanitary napkins, and incontinence pads. Moreover, there has been a recent trend toward decreasing the fiber base material such as pulp and increasing the water-absorbent resin content of the sanitary materials in order to decrease the thickness and improve the performance of the sanitary materials. Thus, there is demand for a further improvement of the absorption properties of the water-absorbent resin. Such demand can be satisfied by improving the absorption properties, more particularly water absorbing capacity and absorption rate. For example, according to a known method for improving the absorption properties, the surface area of the water-absorbent resin is increased. However, if the surface area is increased by simply decreasing the particle diameter of the particulate water-absorbent resin, the liquid permeability in the water-absorbent resin is worsened.
As a method for increasing the surface area without decreasing the particle diameter, for example, Japanese publication of unexamined patent application (Tokukaihei) Nos. 5-237378 and 7-185331, and publication of international application No. WO95/02002 proposed a method for producing a water-absorbent resin in the form of porous particles by causing the particles of the water-absorbent resins to contain cells therein using a blowing agent during polymerization or crosslinking.
Thus, in a suitable method for improving the absorption properties of a water-absorbent resin, a crosslinked hydrogel polymer is formed by aqueous solution polymerization of, for example, a water-soluble ethylenically unsaturated monomer under the presence of a crosslinking agent so that the crosslinked hydrogel polymer includes cells therein, crushed, dried, and then ground. However, particularly, when the crosslinked hydrogel polymer is formed by aqueous solution polymerization of a water-soluble ethylenically unsaturated monomer under the presence of a crosslinking agent so that the crosslinked hydrogel polymer includes cells therein, if the crosslinked hydrogel polymer is crushed by a meat chopper or kneader, the cells in the crosslinked hydrogel polymer are squashed. As a result, the crushed crosslinked hydrogel polymer has a reduced number of cells. In the resultant water-absorbent resin, the liquid guide space that is necessary for the movement of an aqueous liquid cannot be sufficiently ensured because the surface area is decreased by the reduction in the number of cells. Thus, there is a possibility that the permeability and dispersibility of the aqueous liquid are lowered. Hence, a water-absorbent resin produced by the above-mentioned process suffers from problems that the absorption properties such as absorption rate and water absorbing capacity are lowered as well as an increase in the water-soluble component content.
On the other hand, when the above-mentioned method 3 is adopted, it is possible to avoid squashing of the cells in the crosslinked hydrogel polymer during chopping. However, with this method, the productivity is low, and therefore industrial production of water-absorbent resins is almost impossible. Moreover, with this method, since the crosslinked hydrogel polymer has relatively strong adhesion properties, it adheres to the cutting blades of scissors, and therefore the quality of the blades is degraded with time.
Meanwhile, with the above-mentioned method 4, the highly elastic crosslinked hydrogel polymer is deformed when it is pressed against the counter roll, and the crosslinked hydrogel polymer winds itself around the ring-shaped cut edge. Consequently, the crosslinked hydrogel polymer cannot be cut into a desired size, and the operation cannot be performed continuously.
Another method for granulating a mass of crosslinked hydrogel polymer is disclosed in Japanese publication of examined patent application (Tokukohei) No. 3-2042 corresponding to U.S. Pat. No. 4,690,788. In this method, the water-soluble polymer gel is crushed by chewing and cutting it between two rotating roller-type cutters.
However, this crushing method also does not consider polymer gels having cells. When this crushing method is used, in order to crush the polymer gel, first, the polymer gel is chewed between the two roller-type cutters, and cut. After crushing, the polymer gel is extruded through a die. Hence, if the crushed polymer gel contains cells, the cells are squashed in the above-mentioned step, and the number of cells is significantly reduced. Accordingly, the water-absorbent resin produced by this method has a considerably reduced number of cells and a small surface area. Namely, this water-absorbent resin has deteriorated absorption properties. In the above-mentioned situation, there is demand for a process of producing an industrially producible water-absorbent resin with excellent absorption properties.